fix(alerting): populate AlertInstance.rule_snapshot so history survives rule delete

- Add withRuleSnapshot(Map) wither to AlertInstance (same pattern as other withers)
- Call snapshotRule(rule) + withRuleSnapshot in both applyResult (single-firing) and
  applyBatchFiring paths so every persisted instance carries a non-empty JSONB snapshot
- Strip null values from the Jackson-serialized map before wrapping in the immutable
  snapshot so Map.copyOf in the compact ctor does not throw NPE on nullable rule fields
- Add ruleSnapshotIsPersistedOnInstanceCreation IT: asserts name/severity/conditionKind
  appear in the rule_snapshot column after a tick fires an instance
- Add historySurvivesRuleDelete IT: fires an instance, deletes the rule, asserts
  rule_id IS NULL and rule_snapshot still contains the rule name (spec §5 guarantee)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cameleer-server-app/src/test/java/com/cameleer/server/app/alerting/eval/AlertEvaluatorJobIT.java

@@ -196,4 +196,53 @@ class AlertEvaluatorJobIT extends AbstractPostgresIT {
         jdbcTemplate.update("DELETE FROM alert_instances WHERE rule_id = ?", ruleId2);
         jdbcTemplate.update("DELETE FROM alert_rules WHERE id = ?", ruleId2);
     }
+
+    @Test
+    void ruleSnapshotIsPersistedOnInstanceCreation() {
+        // Dead agent → FIRING instance created
+        when(agentRegistryService.findAll())
+                .thenReturn(List.of(deadAgent(Instant.now().minusSeconds(120))));
+
+        job.tick();
+
+        // Read rule_snapshot directly from the DB — must contain name, severity, conditionKind
+        String snapshot = jdbcTemplate.queryForObject(
+                "SELECT rule_snapshot::text FROM alert_instances WHERE rule_id = ?",
+                String.class, ruleId);
+
+        assertThat(snapshot).isNotNull();
+        assertThat(snapshot).contains("\"name\": \"dead-agent-rule\"");
+        assertThat(snapshot).contains("\"severity\": \"WARNING\"");
+        assertThat(snapshot).contains("\"conditionKind\": \"AGENT_STATE\"");
+    }
+
+    @Test
+    void historySurvivesRuleDelete() {
+        // Seed: dead agent → FIRING instance created
+        when(agentRegistryService.findAll())
+                .thenReturn(List.of(deadAgent(Instant.now().minusSeconds(120))));
+        job.tick();
+
+        // Verify instance exists with a populated snapshot
+        String snapshotBefore = jdbcTemplate.queryForObject(
+                "SELECT rule_snapshot::text FROM alert_instances WHERE rule_id = ?",
+                String.class, ruleId);
+        assertThat(snapshotBefore).contains("\"name\": \"dead-agent-rule\"");
+
+        // Delete the rule — ON DELETE SET NULL clears rule_id on the instance
+        ruleRepo.delete(ruleId);
+
+        // rule_id must be NULL on the instance row
+        Long nullRuleIdCount = jdbcTemplate.queryForObject(
+                "SELECT count(*) FROM alert_instances WHERE rule_id IS NULL AND rule_snapshot::text LIKE '%dead-agent-rule%'",
+                Long.class);
+        assertThat(nullRuleIdCount).isEqualTo(1L);
+
+        // snapshot still contains the rule name — history survives deletion
+        String snapshotAfter = jdbcTemplate.queryForObject(
+                "SELECT rule_snapshot::text FROM alert_instances WHERE rule_id IS NULL AND rule_snapshot::text LIKE '%dead-agent-rule%'",
+                String.class);
+        assertThat(snapshotAfter).contains("\"name\": \"dead-agent-rule\"");
+        assertThat(snapshotAfter).contains("\"severity\": \"WARNING\"");
+    }
 }